As an approach to higher densities of optical disks, an optical disk device that uses an optical head, constituted by a condensing system that implements a high numerical aperture by combining an objective lens and a solid immersion lens (hereafter SIL), has been proposed.
In this system (hereafter SIL system), a high refractive index (about 1.8 to 2.0) material is used for the SIL and protective layer of the optical disk, and gap control is performed, so that information is recorded and reproduced using emission light from the SIL generated by bringing the gap between the SIL and protective layer of the optical disk to a micro value, around 25 nm.
FIG. 15 is a basic configuration of the optical system for recording and reproducing information to/from an optical disk using SIL.
In FIG. 15, 101 shows a light source, 103 shows an objective and 104 shows an SIL of which end face 108 has a circular shape, and a light beam 102 emitted from the light source 101 is condensed so that an appropriate spot size is obtained on the recording/reproducing surface 106 of an optical disk 105 by a condensing system comprised of the objective lens 103 and SIL 104.
At this time, a gap 109 between the surface 107 of the optical disk 105 and the end face 108 of the SIL 104 must be held to a micro value, around 25 nm by the above mentioned gap control.
The gap detection for this gap control is performed by converting the sum of the reflected light quantity from the end face 108 of the SIL 104 and the reflected light quantity from the recording/reproducing surface 106 of the optical disk 5, of the light beam 102 that enters the end face 108 of the SIL 104, into a gap detection signal 112 using a detector 111.
The beam splitter 110 is for allowing the reflected light from the end face 108 of the SIL 104 and the reflected light from the recording/reproducing surface 106 of the optical disk 105, of the light beam 102 that enter the end face 108 of the SIL 104, to enter the detector 111.
FIG. 16 is a graph on the relationship of the gap detection signal 112 and the gap 109, and shows characteristics of the incident light quantity (level of normalized gap detection signal 112) that enter the detector 111 with respect to the gap length (length of the gap 109).
In FIG. 16, the abscissa is the gap length (unit: nm) and the ordinate is the level of the gap detection signal 112 detected by the detector 111, that is normalized by the level of the gap detection signal 112 when the gap 109 is sufficiently large.
By performing an appropriate processing on the gap detection signal 112, such as comparing the gap detection signal 112 with an appropriate reference level and amplifying this difference, and driving the actuator 113 using this processing result, the gap 109 can be controlled to be maintained at a desired value.
For example, if the target value of the gap 109 is 25 nm, as mentioned above, the level of the gap detection signal 112 normalized in this case is 0.45 according to FIG. 16, therefore the reference level of the gap control is set to a value corresponding to 0.45.
In the above description, the example of using the actuator 113 only for gap control was used, but the actuator 113 can also be used for tracking control and/or tilt control depending on purpose. Patent Document 1 discloses an example of such gap control.
However the above mentioned gap 109 is extremely small, around 25 nm, so caution is required to avoid collision of the optical disk 105 and the SIL 104, and particularly when the optical disk 105 is relatively tilted with respect to the SIL 104, the possibility of this collision is expected to further increase.
FIG. 17 shows a state where the optical disk 105 is relatively tilted with respect to the condensing system comprised of the objective lens 103 and SIL 104 shown in FIG. 15, and a part of the end face 108 of the SIL 104 contacts the surface 107 of the optical disk 105.
In FIG. 17, if the distance between the center of the end face 108 of the SIL 104 and the surface 107 of the optical disk 105 is the gap 109, of which value is 25 nm, and the current diameter 131 of the end face 108 of the SIL 104 is 40 μm, then an angle formed by the end face 108 of the SIL 104 and the surface 107 of the optical disk 105, that is tilt 130 as an angle formed by the end face 108 of the SIL 104 and the recording/reproducing surface 106 of the optical disk 105 under the state shown in FIG. 17, is 0.07°. This means that the relative inclination of the SIL 104 and the optical disk 105, that is the tilt 130, must always be less than 0.07° so that the SIL 104 and the optical disk 105 do not collide.
The above indicates the necessity to establish tilt control, to make the tilt 130 less than 0.07° before establishing the above mentioned gap control.
Patent Document 2 discloses an example of the tilt control method to prevent the above mentioned problem, and this method includes a step of contacting the SIL with the optical disk in advance.
It is preferable that contact of an optical disk and SIL in an optical disk device, which may cause physical damage to each other, is avoided, and therefore the tilt control method according to Patent Document 2 may cause mutual damage to the SIL and optical disk.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-319160    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-259329